Digital Potentiometer Control Question

Mike 12816

We are attempting to use an AD5220 digital potentiometer programmed by a Basic Stamp to control a curtis 1505 speed control board for an electric vehicle project.· The AD5220 has been wired and programmed using the code from Parallax and works find.· The speed control board requires a 5K pot to vary the speed output and works fine with a hard wired pot.

To eliminate the moveable parts, we tied the grounds from the Basic Stamp board (SumoBot) and the speed control board together and then connected the terminal outputs (A1 and B1) and the wiper (W1) to the terminals on the speed control board.· We only get a constant 2.5 vdc from the terminals that the moter would be connected to and the voltage does not vary.· The voltage on the hardwired pot varies from 0-3vdc.

Attached are the two schematics for the speed control board and the chip.· Does anyone have any ideas on how to eliminate the hardwired potentiometer.

Thanks,

Mike White

stamptrol

I haven't had too much luck with digital pots being interfaced to external equipment.

One issue is that the pot chip supply voltage sets the limit as to the voltage on A, B, W.

Secondly, the current capability is quite low ( 20mA) so your drive may need a bit more.

Two possible solutions have been used in the past. The first is to use a servo motor to adjust the existing mechanical pot.

You might also be able to replace the pot with two light-dependent-resistors and adjust their value by shining a variable intensity light on them.

Cheers,

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Tom Sisk

http://www.siskconsult.com
·

Mike 12816

Thanks for the ideas - the voltage measured on the physical pot went from 0-3 volts so I think the digital pot should be able to handle that. The current may be another issue, but I don't it is.

We already tried using a 360 sail winch servo, but had problems with that and wanted to elimate the brackets and couplings.

So, if anyone else has any ideas that would be greatly appreciated. Otherwise we will go back to the servo.

Thanks again,

Mike

PJAllen

With the 'digi-pot' terminals disconn'd, you should be able to measure/see varying resistance with your ohmmeter (wiper to either terminal.)· If not, why not?

I'd braid the leads between the digi-pot and curtis unit (if > 6 inches away.)

Mike 12816

OK - with the digi pot terminals (wiper to other leg) connected to an ohm meter I only get 2.75k ohms. Running the program that allows you to type in 0-128 does not vary the resistance. I would not think that I could hook up a an external meter to the digi pot and get readings given the internal wiring.

Thanks for the help - I'm still open to more ideas.

Mike 12816

OK - just played around with it again and I am getting a variable resistance with the digi pot running off of the stamp. A one increment step adds about 78 ohms. Reconnect to the speed control board and output voltage does not change. Still hoping.

PJAllen

If you can't measure resistance with an ohmmeter, then it's not much of a pot.· At first it didn't vary and now it does.· Fine.

Onion, onion, onion; peel, peel, peel.

78Ω / step = 10K "pot".· It's supposed to have a 5K, though.· Maybe a 10K will "work" and·maybe it won't "work".· Tried using a 10K pot instead, just for grins and giggles?

W9GFO

"Open wire pot fault feature turns off controller if any throttle wire becomes open circuit."

Maybe the 10K digital pot is causing the problem?

Rich H

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The Simple Servo Tester, a kit from Gadget Gangster.

PJAllen

·and... the line's gone dead.

Hobbyist

Ahh.. I wish this thread had continued to the point of determining if the 10K pot was going to work.. I'm not sure why Mike would pick an AD 5220 instead of an AD 5290. The 5290 has a +/- 15V or 0/30V supply range. My problem still remains that I can't find any 5K digital pots from anybody else that has supply rails that will work with the 24V supply I'm running the Curtis 1505 on. I'm going to have to try paralleling 2 pots and updating both with the same value at the same time if the 5K spec is critical.

My AD 5290 samples arrived yesterday so I haven't got anything rigged up yet. I guess I need to do some tests with fixed resistors and the scope to see for sure if the potential at the A,B, and/or W terminals ever reaches 24V (or if the Curtis has a lower "control circuitry" voltage). There sure isn't a lot of info out there for these old boards.

Here's my test jig at the moment:



The 5290 data sheet is here:

http://www.analog.com/en/digital-to-analog-converters/digital-potentiometers/ad5290/products/product.html

Here's a pic of the project at this (very early) stage:



Oh ya.. Hi Parallax forum.. this looks like a cool site.. hope to be a valuable member

Jim Patek

Hobbyist
Any word on how the AD5290 worked out? I have a similar problem needing a 5k 24v digital pot, so I'd really like to know what happened here.

Mike Green

There are no digital pots that can handle voltages like what you want. Most digital pots can handle voltages to 5V or 0V when powered by +5V/0V. Some of them only work up to 3.3V. They're simply neither designed nor capable of handling higher voltages. If the pot is being used as a voltage divider on a 24V supply, you could use an op-amp rated for 24V or higher and drive that from a digital to analog converter controlled by the Stamp. Another solution that can work nicely is to use a servo motor mechanically coupled to a 5K pot. Most pots cover a 270 degree range and you can get servos that will move through that.

It may be that the pot is not expected to be subject to voltages like 24V. It may be used in a low-voltage circuit where everything is 5V or less. That could work nicely with an ordinary digital pot powered by 5V. You need to get more information on your circuit.

Alan Bradford

Lets go outside the box for a bit...

Forget the Pot as it is the control being replaced.
· 1: Measure the voltage from pin6 (High) to pin 4 (Low)
If it is 24 volts· or maybe a bit less then most likely it is a voltage divider generating a reference voltage for the Throttle speed.

· 2: If so you can take your 5 volt digital pot 5k, 10k, 100k· whatever, and control transistor amp·with 4:1 gain.
The Amp·output can go to pin 5 (Wiper) and drive the throttle circuit.
I think there are some op am ps that can work at 24 volts.

· If the pot voltages are diffrent or changing then look at the circuit board and try to draw the circuit used.
This will assist in figuring out a way to control the input stage with nomoving parts.

Good Luck,

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Alan Bradford ·N1YMQ

Plasma Technologies
Canaan NH 03741
www.plasmatechnologies.com

stamptrol

The other point is that many drives allow a direct voltage input rather than being stuck with a pot. Checking the connection to the pot's wiper may show that the input required is really a voltage.

As well, many will allow a choice of 0-5 volts or 0-10 volts so its possible to use a simple PWM output and filter to get a voltage the drive will respond to.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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